1. Field of the Invention
The present invention relates generally to oscillation circuits, and is directed to an improvement in an oscillation circuit which comprises an active circuit portion and a surface acoustic wave resonance device of the single port type connected with the active circuit portion and in which a signal feedback from the active circuit portion to the surface acoustic wave resonance device of the single port type is carried out to raise an oscillating operation at an oscillation frequency determined in accordance with the resonance frequency of the surface acoustic wave resonance device of the single port type.
2. Description of the Prior Art
There has been proposed an oscillation circuit using a surface acoustic wave resonance device for producing an oscillation output signal having a frequency in the range of megahertz (MHz), for example, from tens to hundreds MHz. Surface acoustic wave resonance devices are classified into single port and double port types. A surface acoustic wave resonance device of the single port type is represented by means of an equivalent circuit shown in FIG. 1 and a surface acoustic wave resonance device of the double port type is represented by means of an equivalent circuit shown in FIG. 2.
The equivalent circuit representing the surface acoustic wave resonance device of the single port type as shown in FIG. 1 comprises a series connection of a resistant element 11, an inductive element 12 and a capacitive element 13 and a capacitive element 14 connected in parallel with the series connection of the resistant element 11, inductive element 12 and capacitive element 13 so as to correspond to a stray capacitance accompanying that series connection. The equivalent circuit representing the surface acoustic wave resonance device of the double port type as shown in FIG. 2 comprises a series connection of a resistant element 15, an inductive element 16 and a capacitive element 17, a couple of inductive elements 18 and 19 in which the inductive element 18 is connected in series with the series connection of the resistant element 15, the inductive element 16 and the capacitive element 17, a capacitive element 20 connected in parallel with the series connection of the resistant element 15, the inductive element 16, the capacitive element 17 and the inductive element 18 so as to correspond to a stray capacitance accompanying that series connection, and a capacitive element 21 connected in parallel with the inductive element 19 so as to correspond to a stray capacitance accompanying the inductive element 19.
As understood with the equivalent circuits shown in FIG. 1, the surface acoustic wave resonance device of the single port type can be treated in the same manner as a quartz resonator and therefore it is possible to form an oscillation circuit having a relatively simple structure with use of the surface acoustic wave resonance device of the single port type. In case of an oscillation circuit making use of the surface acoustic wave resonance device of the single port type, an active circuit portion including, for example, transistors and a signal feedback circuit is provided and both ends of the surface acoustic wave resonance device of the single port type are connected with the active circuit portion, so that a signal feedback from the active circuit portion to the surface acoustic wave resonance device of the single port type is carried out to raise an oscillating operation at an oscillation frequency determined in accordance with the resonance frequency of the surface acoustic wave resonance device of the single port type.
However, since the surface acoustic wave resonance device of the single port type is subjected to a harmful influence of stray capacitance to a larger extent more than the surface acoustic wave resonance device of the double port type, it may be difficult for an oscillation circuit having a fundamental structure comprising an active circuit portion and a surface acoustic wave resonance device of the single port type connected with the active circuit portion to oscillate properly at a relatively high frequency range. Accordingly, it is usual, for the oscillation circuit having the fundamental structure comprising the active circuit portion and the surface acoustic wave resonance device of the single port type connected with the active circuit portion, to oscillate at a relatively low frequency range such as, for example, at the frequency range from 40 to 300 MHz.
Assuming in the equivalent circuit shown in FIG. 1, R represents the resistance of the resistant element 11, L represents the inductance of the inductive element 12, C represents capacitance of the capacitive element 13, Cs represents the capacitance of the capacitive element 14, namely, a stray capacitance, and Fo represents the resonance frequency of the surface acoustic wave resonance device of the single port type, namely, an oscillation frequency, under a condition in which the stray capacitance Cs is so small as to be substantially negligible, a value Q which indicates sharpness in resonance of the surface acoustic wave resonance device of the single port type is shown by the following equations. EQU Q=Z/R EQU Z=2.multidot..pi..multidot.Fo.multidot.L-1/(2.multidot..pi..multidot.Fo.mul tidot.C)
The oscillation circuit comprising the active circuit portion and the surface acoustic wave resonance device of the single port type connected with the active circuit portion is operative to oscillate when Z has a positive value. In the oscillation circuit comprising the active circuit portion and the surface acoustic wave resonance device of the single port type connected with the active circuit portion and put in its oscillating operation, Z increases monotonously in response to increase in the oscillation frequency and the value Q also increases monotonously.
To the contrary, under a condition in which the stray capacitance Cs is so large as not to be substantially neglected, the oscillation frequency is reduced to be lower than the resonance frequency Fo of the surface acoustic wave resonance device of the single port type which is obtained under the condition in which the stray capacitance Cs is so small as to be substantially negligible and the value Q indicating sharpness in resonance of the surface acoustic wave resonance device of the single port type is also reduced to be lower than that under the condition in which the stray capacitance Cs is so small as to be substantially negligible. That is, the effective value of the value Q indicating sharpness in resonance of the surface acoustic wave resonance device of the single port type is reduced due to the harmful influence of the stray capacitance, and therefore, the oscillation circuit comprising the active circuit portion and the surface acoustic wave resonance device of the single port type connected with the active circuit portion may be unable to oscillate properly at the relatively high frequency range. Consequently, the oscillation circuit proposed previously to have the fundamental structure comprising the active circuit portion and the surface acoustic wave resonance device of the single port type connected with the active circuit portion is accompanied with a disadvantage or problem that an oscillation output signal at the relatively high frequency can not be obtained.